1. Field of the Invention
The present invention relates to a frequency conversion circuit which constitutes an essential stage of a transmitter and of a receiver in a radio communication device.
2. Description of the Related Art
In a field of the personal communication wherein the number of subscribers will rapidly increase, it is requested to develop a frequency conversion circuit with improved linear property, as soon as possible. Generally, the intermodulation distortion (IM) property of a receiver front end is limited by that of the mixer having worse IM property than the other stages in the receiver. Therefore, improving the mixer's IM property can do much to improve the whole IM property of the receiver front end and also to improve dynamic range.
A drain LO injection mixer is known as a mixer having relatively good IM property and is described for example in "An Unusual Microwave Mixer", by G. Tomasetti, Proc. 16th Microwave Conf., 754, (1987). According to this mixer, the LO (Local Oscillator) signal and the IF (Intermediate Frequency) signal are fed to and taken from the drain of the GaAs FET while the RF (Radio Frequency) signal is fed to the gate of this FET.
The input impedance of the FET in this mixer almost depends upon the gate-source capacitance when this mixer is used at the mobile communication frequency of around 1 GHz. For example, in case of the GaAs FET with the gate width of 100 .mu.m, the input impedance depending upon the gate-source capacitance will be about 2 k.OMEGA. (kilo-ohms order). Since the gate voltage amplitude of the FET is determined only by its threshold voltage and by its Schottky gate voltage, dynamic range for RF signal will be narrowed if the input impedance is great, resulting the IM property to be extremely made worse than that at several tens GHz.
The most commonly used method for broadening the dynamic range is to reduce the input signal level to the mixer. However, this reduction of the input signal level will result a low level output signal from the mixer causing a heavy burden to an amplifier in the following stage. Another method for broadening the dynamic range is to increase the gate width of the FET so as to reduce its input impedance. However, this latter method also has problems as that the conversion gain decreases by 3 dB and double DC power will be consumed at the FET if its gate width is doubled. In order to compensate the decreasing conversion gain, it is necessary to increase the LO signal level to the drain.
A resistive mixer has been employed for suppressing IM as described in "A GaAs MESFET Mixer with Very Low Intermodulation", by S. A. Mass, IEEE Trans. MTT, Vol.35, 425 (1987). According to this mixer, a small RF signal is fed to the drain of the FET and a large LO signal is fed to its gate, and then the LO signal modulates the junction conductance at the LO frequency by using a switching operation of the FET. Since the channel resistance of this FET is substantially linear, the IM can be suppressed enough. However, because the FET operates only in the linear region with no amplifying operation, this resistive mixer provides a very small conversion gain and a poor noise property.
As aforementioned, the conventional frequency conversion circuits have sacrificed the conversion gain (or output frequency signal power) and the noise figure for improvement of the IM property.